Human skeletal muscle exhibits combined properties, in terms of active strain, active stress, active strain rate, variable stiffness, power to mass ratio and bandwidth, which classical actuator technologies do not match. Hunter I. W. and Lafontaine S., “A Comparison of Muscle with Artificial Actuators”, Technical Digest IEEE Solid State Sensors and Actuators Workshop, pp. 178–185, 1992. On the other hand, conducting polymer materials offer properties enabling the creation of biomimetic artificial muscles. Polypyrrole-based actuators, for example, generate forces per cross-sectional area that are up to two orders of magnitude greater than human muscle (40 MPa) with higher power to mass ratios (150 W/kg). In addition, these actuators require low drive voltages and offer typically limited active strain (2%) and limited active strain rate (1%/s). Madden J. D., Madden P. G. and Hunter I. W., “Polypyrrole Actuators: Modeling and Performance”, Electroactive Polymer Actuator and Devices Conference, SPIE 8th Annual International Symposium on Smart Structures and Materials, Newport Beach, Calif., 2001; Madden J. D., Madden P. G. and Hunter I. W., “Conducting Polymer Actuators As Engineering Materials”, Smart Structures and Materials 2002: Electroactive Polymers Actuators and Devices, Yoseph Bar-Cohen, Editor, Proceedings of the SPIE, Vol. 4695, pp. 176–190, 2002.
Actuation in conducting polymers such as polypyrrole or polyanilines is based on electrochemical oxidation and a resulting diffusion and intercalation of ionic species into the polymer bulk film. Baughman R. H., Shacklette S. W., Plichta E. J. and Becht C., “Electromechanical Actuators Based on Conducting Polymers”, Molecular Electronics, pp. 267–289, 1991. This ionic intercalation process arises to maintain electro-neutrality during the oxidation process, leading to significant volume changes. In addition, accommodation of these ions and their associated solvated species is favored by the weak polymer interchain interactions compared to the modulus along the polymer molecular backbone. As a result strains on the order of 2% are produced upon electroactivation. It is conceivable that ultimately the maximum strain achievable in these “classical” conducting polymer actuators is going to be limited by the inability of the polymer molecular backbone to significantly change length to accommodate further ions.
Molecular actuators, i.e. biological or synthetic molecular systems performing work upon consumption of energy have triggered great interest in various fields such as biology, chemistry, chemical engineering and mechanical engineering. Soong R. K., Bachand G. D., Neves H. P., Olkhovets A. G., Craighead H. G. and Montemagno C. D., “Powering an Inorganic Nanodevice with a Biomolecular Motor”, Science, Vol. 290, pp. 1555–1558, 2000; Astumian R. D., “Making Molecules into Motors”, Scientific American, pp. 57–64, July 2001; Ballardini R., Balzani V., Credi A., Gandolfi M. T. and Venturi M., “Artificial Molecular-Level Machines: Which Energy to Make them Work?”, Accounts of Chemical Research, Vol. 34, pp. 445–455, 2001; Collin J. P., Dietrich-Buchecker C., Jimenez-Molero M. C. and Sauvage J. P., “Shuttles and Muscles: Linear Molecular Machines Based on Transition Metals”, Accounts of Chemical Research, Vol. 34, pp. 477–487, 2001. Various biological machines such as the ATP synthase, or kinesins have been studied extensively; Synthetic non-conducting polymer molecules embedding metal complexes that exhibit electron-induced (redox) chirality have been created; Molecular (robotic) grippers made from resorcin[4]arene have been demonstrated, to cite just a few. Zahn S. and Canary J. W., “Electron-Induced Inversion of Helical Chirality in Copper Complexes of N,N-Dialkylmethionines”, Science, Vol. 288, pp. 1404–1407, 2000; Yamakoshi Y., Schlitter R. R., Gimzewski J. K. and Diederich F., “Synthesis of Molecular-Gripper Type Dynamic Receptors and STM-imaging of Self-Assembled Monolayers on Gold”, Journal of Materials Chemistry, Vol. 11, pp. 2895–2897, 2001. A possible mechanism of molecular actuation using cyclooctatetrathiophene conducting polymers has also been recently investigated by Marsella and colleagues. Marsella M. J. and Reid R. J., “Toward Molecular Muscles: Design and Synthesis of an Electrically Conducting Poly[cyclooctatetrathiophene]”, Macromolecules, Vol. 32, pp. 5982–5984, 1999. Such materials with large contractions and great strength will lead to many useful applications.